Two cavity laser

ABSTRACT

A laser system includes a single laser material with means defining a first optical cavity for producing regeneration of radiation of a first oscillating mode and additional means defining a second optical cavity for the same laser material providing regeneration of radiation of a second oscillating mode. By this arrangement, distinct first and second output beams of different mode oscillations and of the same frequency are provided from a single laser material.

United States Patent Schulthess et al.

[151 3,663,890 51 May 16, 1972 [54] TWO CAVITY LASER 72 Inventors: CarlWilliam Schulthess, Pomona; Eduard Gregor, Pacific Palisades, both ofCalif.

Union Carbide Corporation, New York, N.Y.

[22] Filed: Apr. 27, 1970 [21] Appl.No.: 31,978

[73] Assignee:

s2 U.S.Cl. ..331/94.s s11 1nt.C| ..H0ls3/00 [58] FieldofSearch...33l/94.5;350/l69,172

[ References Cited UNITED STATES PATENTS Miller ..33 l/94.5'X

Schneider et al ..33 l/94.5 Holbrook et a1 ..350/172 X PrimaryExaminerRonald L. Wibert Assistant Examiner-Conrad ClarkAttorney-Pastoriza & Kelly [57] ABSTRACT 8 Claims, 4 Drawing Figures 24LASER 55 E Q 5 HEAD E E 5 sw E TWO CAVITY LASER This invention relatesgenerally to laser systems and more particularly to a novel laseroscillator having two optical cavities for providing distinct outputbeams of radiation of different oscillating modes.

BACKGROUND OF THE INVENTION In certain laser applications, it is oftendesirable to split the output laser beam to provide two distinct beamsof the same frequency for various operations. For example, in pulsedreflection holography, an illuminating beam is required for lighting theobject of which a three dimensional film picture is to be made andsimultaneously a reference beam of the same frequency as theilluminating beam is directed towards a film plane receiving reflectedlight from the object. Interference takes place between the reflectedilluminating beam from the object and the reference beam bothconstructively and destructively to create the hologram in question.

In the above specific example of holography, the reference beam ispreferably of a single mode or TEM oscillating mode while theilluminating beam may be a multimode oscillation. Also, the illuminatingbeam has from to times as much energy as the reference beam. Inaccordance with present methods, the output from a laser such as apulsed ruby oscillator is in the form of a single mode oscillationreferred to as a TEM,, mode. This single mode is accomplished byincorporating an aperture plate in the optical cavity of the laserwherein the normally present multimode oscillations are blocked frombeing coupled out of the system, the single mode being essentiallydefined by the aperture in the plate. This single mode is then passedthrough a beam splitter which is designed to reflect from one-tenth toone-twentieth the beam energy while passing from 90 to 95 percent of thebeam. The larger energy beam passed through the beam splitter may thenbe utilized to illuminate the object of which a hologram is to be made.A diffuser plate is preferably employed to render more uniform theillumination of the object. The reference beam reflected from the beamsplitter in turn is directed towards the film plane on which thehologram is to be formed.

It will be evident from the above description that much useful energy inthe laser system itself is lost since the output beam from the lasersystem itself is of considerably reduced energy as a consequence of theaperture plate. While there ultimately results two distinct beams bypassing the output through the beam splitter, to the extent thatmultimode oscillations in the laser optical cavity are lost the overallsystem is inefficient.

BRIEF DESCRIPTION OF THE PRESENT INVENTION In accord with the presentinvention, a laser system is provided wherein two distinct output beamscan be coupled directly out of the laser system itself so thatoscillating energy in the system is utilized to maximum efficiency. Byproviding the two distinct output beams in the form of a multimodeoscillating radiation and a single mode TEM oscillating radiation, thesystem is well suited to the holographic operations.

Coupling out of two distinct laser beams from a single laser material isachieved by providing two optical cavities cooperating with the samelaser oscillator. Thus, for the specific holographic operation, meansare provided defining a first optical cavity for the material providingregeneration of radiation of a first oscillating mode which mayconstitute the multimode illuminating beam and means are provideddefining a second optical cavity for the same laser material providingregeneration of radiation of a second oscillating mode which might bethe TEM single mode beam for reference purposes. These first and secondoutput beams of different mode oscillations are of the same frequency.

The essence of the means defining the first and second optical cavitiestakes the form of an aperture plate having a central aperture andsurrounding 100 percent reflecting mirror on one surface. This plate ispositioned to intercept laser radiation from one end of the lasermaterial such that its reflecting surface passes a portion of theradiation back through the laser material. A first end mirror interceptsthis radiation from the other end of the laser material and reflects atleast a portion of the radiation back through the material to thereflecting surface of the aperture plate. The reflecting surface of theaperture plate and first end mirror thus define a first optical cavityfor the laser material.

A second end mirror which is partially transmissive is positioned tointercept laser radiation passing through the aperture and reflect aportion of the radiation back through the aperture to the one end of thelaser material for further reflection back through the material from thefirst end mirror. This partially transmissive second end mirror and thefirst end mirtor thus define the second optical cavity, the second endmirror serving to couple the second oscillating mode radiation out ofthe system.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of theinvention will be had by referring to the accompanying drawings inwhich:

FIG. 1 is a highly schematic showing of a prior art laser systemproviding two output beams for holographic operatrons;

FIG. 2 is a schematic showing of a first embodiment of a laser system inaccord with the present invention for providing suitable output beamswhich could be substituted for the system of FIG. 1 for holographicoperations;

FIG. 3 is a schematic showing of a second embodiment of the invention;and,

FIG. 4 is a schematic showing of a third embodiment.

DETAILED DESCRIPTION OF THE INVENTION Referring first to FIG. 1 there isshown a typical prior art laser system for making holograms. As shown,this system includes a laser oscillator which may take the form of aruby rod 10 surrounded by a helical flashlamp 11 powered from a source12. First and second end mirrors l3 and 14 define an optical cavity forthe regeneration of radiation from the crystal 10. A Q switch indicatedat 15 is provided for the generation of giant pulses and may take theform of a Kerr or Pockels cell or may be of a passive type such as anorganic dye.

In order to provide a single mode oscillating radiation beam, anaperture plate 16 is disposed in the cavity as shown such that only asingle mode oscillation can pass therethrough. The end mirror 14 ispartially transmissive to couple the single mode oscillation from thelaser system.

In order to provide an illuminating beam and reference beam forholographic work from the system of FIG. 1, there is provided a beamsplitter 17 through which from to percent of the beam energy passes to adiffusion plate 18 and thence to an object 19 of which a hologram is tobe made. The illuminating light reflected from the object 19 strikes afilm plane 20 as indicated.

The reflected portion of the output beam from the beam splitter 17passes to a mirror 21 which directs this reference beam onto the filmplane 20 wherein difraction patterns are set up by interference with thereflected illuminating beam from the object to provide the hologram. Theilluminating and reference beams from the beam splitter 17 aredesignated at 22 and 23 and normally the illuminating beam 22 will havefrom 10 to 20 times as much energy as the reference beam 23.

Because of the relatively small amount of energy in the original outputbeam from the laser system as a consequence of the use of the apertureplate, laser amplifiers (not shown) would ordinarily be provided beforepassing said beam through the diffusion plate to illuminate the object.For example, in the typical arrangement illustrated in FIG. 1 theilluminating beam 22 prior to any amplification might have an energy ofthe order of 50 X 10* joules. If the laser amplifier utilized as amaximum gain of 4, the maximum energy from such amplifier would be lessthan 0.2 joules.

Referring now to FIG. 2 there is shown a first embodiment of the presentinvention for providing an illuminating and reference beam from a singlelaser material which could be substituted for the system of FIG. 1thereby avoiding the need of the beam splitter 17 exterior of the lasersystem. Thus, there is provided a laser head designated by the box 24which would include the laser material and helical flashlamp such asillustrated in FIG. 1. First and second end mirrors 25 and 26 define anoptical cavity in a manner similar to that shown in FIG. 1 and a Qswitch is provided to enable the generation of giant laser pulses.

In accord with the invention, there is provided in the first embodimentof FIG. 2 an aperture plate 27 having a 100 percent reflecting surface28 surrounding the central aperture opening. This aperture plate ispositioned between the laser material in the head 24 and the second endmirror 26 and disposed at a 45 angle. A third mirror 29 which ispartially transmissive is disposed to intercept radiation from thereflecting surface 28 on the aperture plate 27 and direct this radiationback to the reflecting surface 28 and thence through the laser head 24to the first end mirror 25. For the multimode radiation reflected fromthe reflecting surface 28 of the aperture, there is thus defined a firstoptical cavity between the mirrors 25 and 29, this cavity being Lshaped. A first oscillating mode beam which in the example chosen is amultimode oscillation is coupled out of the system through the thirdmirror 29 as indicated by the arrow 30. The energy distribution for thisfirst beam is indicated by the wave form adjacent to the arrow 30.

A second optical cavity for the laser material in the head 24 is definedbetween the first and second end mirrors 25 and 26 and providesregeneration for a second oscillating mode which in the example takenfor illustrative purposes is a single mode TEM oscillation. This mode isprovided by radiation passing through the central aperture of the plate27 which radiation is ultimately coupled out of the second end mirror 26as indicated by the arrow 31. The single mode characteristic of theradiation is indicated by the wave form adjacent to the arrow 31.

With the foregoing arrangement, suitable 100 percent mirrors may beprovided to direct the respective first and second output beams 30 and31 to an object and film plane to make up a hologram as described inFIG. 1. It will be noted, however, that substantially all of the energygenerated in the laser head is utilized in providing these first andsecond beams. This consequence is a direct result of providing twocavities cooperating with the single laser material.

Referring now to FIG. 3 there is shown an alternative or secondembodiment of the two cavity laser system. In this embodiment, thereagain is provided a laser head 32 with a first end mirror 33 and asecond end mirror 34 positioned to received a portion of radiationreflected from a beam splitter 35 disposed between the laser head 32 anda third mirror 36. An aperture plate 37 in turn is disposed between thesecond mirror 34 and beam splitter 35. This aperture plate includes a100% reflecting surface 38 surrounding its central aperture such that aportion of the radiation reflected from the beam splitter 35 isrereflected by the mirror surface 38 and beam splitter 35 back throughthe laser head 32 to the first end mirror 33. A portion of thismultimode oscillation passing through the beam splitter 35 is reflectedfrom the mirror 36 back to the first end mirror 33. A 100 percentreflecting mirror 39 is positioned on the side of the beam splitter 35opposite to that of the aperture plate 37 for directing radiation fromthe beam splitter 35 back towards the beam splitter as indicated.

The portion of radiation reflected from the beam splitter 35 whichpasses through the central aperture is partially reflected by the secondend mirror 34 back up through the aperture to the beam splitter 35 andthen through the laser head 32 to the first end mirror 33. The secondoptical cavity in the arrangement of FIG. 3 is thus of L shape. In thissystem, the first multimode radiation output beam is coupled out of thesystem from the third mirror 36 as indicated at 40 and the second singlemode oscillating radiation is coupled out of the system through thesecond end mirror 34 as at 41. Substantially all of the generatedradiation energy in the laser system is utilized in the two outputbeams.

In the embodiment of FIG. 4, there is shown a somewhat simplified systemover those illustrated in FIGS. 2 and 3. In this embodiment, there isagain provided a laser head 42 with first and second end mirrors 43 and44 together with the Q switch as shown. An aperture plate 45 ispositioned to intercept radiation from one end of the laser head 42 andincludes a percent reflecting surface 46 surrounding the centralaperture. The first optical cavity for the laser is defined between the100 percent reflecting surface 46 of the aperture plate 45 and the firstend mirror 43. The second optical cavity for the single oscillating modesecond beam is defined between the first and second end mirrors 43 and44, this portion of the radiation passing through the central apertureof the plate 45. The first multimode oscillating beam is coupled out ofthe system through the first end mirror 43 as indicated by the arrow 47and the second output beam constituting the single mode oscillation iscoupled out of the second end mirror 44 as indicated at 48, these firstand second end mirrors both being partially transmissive.

In both the embodiments of FIGS. 3 and 4, additional external 100percent reflecting mirrors may be provided for directing the respectiveoutput beams in proper directions for holographic operations. Also,amplifiers (not shown) may be provided external of the laser system forproviding increased energy outputs.

OPERATION In the operation of the system of FIG. 2, the laser head 24 isrepetitively light pumped and in cooperation with the Q switch and firstand second optical cavities defined respectively between the first andsecond end mirrors and first and third end mirrors will result in theregeneration of two distinct oscillating modes derived from the samelaser material in the laser head. The first oscillating mode in theexample chosen constitutes a multimode oscillating radiation generatedbetween the first end mirror 25 and third end mirror 29 in cooperationwith the reflecting surface 28 on the aperture plate 27 defining the Lshaped first optical cavity. This radiation, as described, is coupledout of the system as indicated at 30 through the third end mirror 29.The second output beam of single mode oscillating radiation is generatedbetween the first and second end mirrors 25 and 26, this radiation beingconfined to that portion which can pass through the central aperture inthe plate 27. In an actual embodiment, the aperture plate 27 andreflecting surface 28 is in the form of an eliptically shaped diagonal.If the minor axis is one inch to accomodate a l inch diameter beam, themajor axis would be l.4l inches for the 45 disposition illustrated. Thecentral aperture might typically have a diameter of 2 millimeters. Withthis arrangement, the first multimode output beam would have an energyof approximately 1 joule and a single laser amplifier (not shown) wouldhave an output of about 4 joules providing an improvement factor of 20over the system described in FIG. 1.

In the embodiment of FIG. 3, a combination beam splitter and apertureplate is utilized, the first output beam being taken from the partiallytransmissive mirror 36 which mirror cooperates with the first end mirror33 to provide a first optical cavity. The second single mode oscillatingradiation beam is confined to the L shaped cavity and takes placebetween the first and second end mirrors 33 and 34 as describedheretofore.

In the operation of the third embodiment illustrated in FIG. 4, only twoend mirrors 43 and 44 are provided but each are made partiallytransmissive to couple the beams respectively out of the laser system.

It will be noted that all of the embodiments include the essentialfeature of an aperture plate with one surface fully 100 percentreflecting surrounding the central aperture opening. In essence, thisprovides the separation of distinct oscillating modes which may then becoupled out of the system by the various mirror and partiallytransmissive mirror arrangements described.

From the foregoing description, it will be evident that the presentinvention has provided an improved laser system particularly well suitedfor use in holography wherein the illuminating beam may be of themultimode oscillating type whereas the reference beam requirespreferably a single mode output. By providing two cavities cooperatingwith a single laser material, the desired multi and single modeoscillations may be coupled out of this system with maximum efficiency.

Although the invention has been described with specific reference toholography and the example of the laser material itself being given asruby, it should be understood that the two cavity laser arrangement canbe used with the other lasing media including neodymium-glass andneodymium-YAG. Further, there will occur to those skilled in the artother uses for the resulting output beams aside from holography.

What is claimed is:

1, A laser system comprising: a single laser material; means includingan aperture plate having a central aperture and a 100 percent reflectingsurface surrounding said aperture to define part of a first opticalcavity for said material providing regeneration of radiation of a firstoscillating mode reflected bysaid aperture plate; means defining asecond optical cavity for said same laser material providingregeneration of radiation of a second oscillating mode passing throughthe aperture in said aperture plate; means for coupling out of saidsystem radiation of said first oscillating mode; and means for couplingout of said system radiation of said second oscillating mode wherebydistinct first and second output beams of different mode oscillationsand the same frequency are provided from said single laser material.

2. A system according to claim 1, in which said state is positioned tointercept laser radiation from one end of said laser material such thatits reflecting surface passes a portion of said radiation back throughsaid laser material; a first end mirror positioned to interceptradiation from the other end of said laser material and reflect at leasta portion of said radiation back through said material to saidreflecting surface of said aperture plate, said means defining a secondoptical cavity including a second end mirror which is partiallytransmissive positioned to intercept laser radiation passing throughsaid aperture and reflect a portion of said radiation back through saidaperture to said one end of said laser material for further reflectionback through said material from said first end mirror and thence backthrough said aperture to said second end mirror, said second oscillatingmode being coupled out of said system through said second end mirror.

3. A system according to claim 2, in which said first optical cavityincludes a third end mirror which is partially transmissive positionedto intercept at least a portion of said first oscillating mode ofradiation, said first oscillating mode being coupled out of said systemthrough said third end mirror.

4. A system according to claim 3, in which said first end mirror ispercent reflecting, said aperture plate being positioned between saidone end of said laser material and said second end mirror and disposedat a 45 angle with respect to the direction of radiation through saidlaser material, said third end mirror receiving and returning said firstoscillating mode of radiation from said reflecting surface of saidaperture plate to define an L shape for said first optical cavity.

5. A system according to claim 3, including a beam splitter positionedbetween said one end of said laser material and said third end mirrorand disposed at a 45 angle with respect to the direction of radiationthrough said laser material, said aperture plate being positioned toreceive radiation reflected by said beam splitter and return radiationin said first oscillating mode, said second end mirror returningradiation of said second oscillating mode to said beam splitter todefine an L shape for said second optical cavity; and a 100 percentreflecting mirror positioned to the side of said beam splitter oppositeto that of said aperture plate to return radiation to said beamsplitter.

6. A system according to claim 2, in which said first end mirror ispartially transmissive to couple out radiation of said first oscillatingmode from said first optical cavity.

7. A system according to claim 2, in which said radiation of a firstoscillating mode includes additional oscillating modes to define amultimode output for the first output beam from said first opticalcavity, said radiation of a second oscillating mode comprising a singleTEM for the second output beam from said second optical cavity.

8. A system according to claim 7, in which the energy of said firstoutput beam is approximately from 10 to 20 times the energy of saidsecond output beam.

i UNI ED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No. 3,663 890 Dated Ma 16, 1972 n t fls) Carl William Sohulthess and EduardGregor It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

Column 5, line 35 Delete-"state" and substitute -plate-- Signed andsealed this 12th day of September 1972.

(SEAL) Attest:

ROBERT GOT'ISCHALK g. uscoMM-Dc 6O376-P69 "1 U.5 GOVERNMENT PRINTINGOFFICE I969 0-366-334 F ORM PO-l050 (10-69)

1. A laser system comprising: a single laser material; means includingan aperture plate having a central aperture and a 100 percent reflectingsurface surrounding said aperture to define part of a first opticalcavity for said material providing regeneration of radiation of a firstoscillating mode reflected by said aperture plate; means defining asecond optical cavity for said same laser material providingregeneration of radiation of a second oscillating mode passing throughthe aperture in said aperture plate; means for coupling out of saidsystem radiation of said first oscillating mode; and means for couplingout of said system radiation of said second oscillating mode wherebydistinct first and second output beams of different mode oscillationsand the same frequency are provided from said single laser material. 2.A system according to claim 1, in which said state is positioned tointercept laser radiation from one end of said laser material such thatits reflecting surface passes a portion of said radiation back throughsaid laser material; a first end mirror positioned to interceptradiation from the other end of said laser material and reflect at leasta portion of said radiation bAck through said material to saidreflecting surface of said aperture plate, said means defining a secondoptical cavity including a second end mirror which is partiallytransmissive positioned to intercept laser radiation passing throughsaid aperture and reflect a portion of said radiation back through saidaperture to said one end of said laser material for further reflectionback through said material from said first end mirror and thence backthrough said aperture to said second end mirror, said second oscillatingmode being coupled out of said system through said second end mirror. 3.A system according to claim 2, in which said first optical cavityincludes a third end mirror which is partially transmissive positionedto intercept at least a portion of said first oscillating mode ofradiation, said first oscillating mode being coupled out of said systemthrough said third end mirror.
 4. A system according to claim 3, inwhich said first end mirror is 100 percent reflecting, said apertureplate being positioned between said one end of said laser material andsaid second end mirror and disposed at a 45* angle with respect to thedirection of radiation through said laser material, said third endmirror receiving and returning said first oscillating mode of radiationfrom said reflecting surface of said aperture plate to define an L shapefor said first optical cavity.
 5. A system according to claim 3,including a beam splitter positioned between said one end of said lasermaterial and said third end mirror and disposed at a 45* angle withrespect to the direction of radiation through said laser material, saidaperture plate being positioned to receive radiation reflected by saidbeam splitter and return radiation in said first oscillating mode, saidsecond end mirror returning radiation of said second oscillating mode tosaid beam splitter to define an L shape for said second optical cavity;and a 100 percent reflecting mirror positioned to the side of said beamsplitter opposite to that of said aperture plate to return radiation tosaid beam splitter.
 6. A system according to claim 2, in which saidfirst end mirror is partially transmissive to couple out radiation ofsaid first oscillating mode from said first optical cavity.
 7. A systemaccording to claim 2, in which said radiation of a first oscillatingmode includes additional oscillating modes to define a multimode outputfor the first output beam from said first optical cavity, said radiationof a second oscillating mode comprising a single TEMoo for the secondoutput beam from said second optical cavity.
 8. A system according toclaim 7, in which the energy of said first output beam is approximatelyfrom 10 to 20 times the energy of said second output beam.